It was surely the bombshell study
of the year. Obesity, far from endangering health, was
associated with lower overall mortality than being underweight
or even of normal weight, claimed a CDC report published
last month in the Journal of the American Medical
Association.
With more than half a million patient-years
of follow-up collected by the National Center for Health
Statistics, this is the mother of all data sets in obesity
research. Most of it was analyzed before, by the same
team, in a 2005 study that reached similar conclusions.
But this time they delved deeper into particular causes
of death, to find where the costs and benefits of extra
weight lay.
CVD
DEATH DECLINE
Perhaps the most jarring finding was the total absence
of extra cardiovascular risk in the overweight category,
representing a body mass index between 25 and 30. Even
though these subjects were at elevated risk of diabetes,
that did not translate into extra cardiovascular deaths.
In fact there was a nonsignificant trend towards reduced
CVD mortality in overweight people. The biggest clue
was under our noses all along. America has continued
to see falling cardiovascular mortality even as obesity
rockets.
The authors conclude that an astonishing
107,000 overweight Americans survived the year 2004
who would have died from a non-cancerous, non-cardiovascular
condition had they been of normal weight. In fact, their
data suggests that the overall number of American lives
saved by being overweight in 2004 was greater than the
number of American lives lost through being obese that
same year. The biggest survival benefit was seen in
chronic respiratory diseases, but the overweight were
also better at overcoming acute infection. The overweight
were less likely than normal-weight people even to die
from injury, suicide or homicide.
On balance, the billions of extra
pounds carried by the citizens of the world's fattest
country are bringing a clear health benefit.
Findings so jarringly out of synch
with conventional wisdom were sure to raise a stir,
but the critics of the team's previous research - who
pelted JAMA with letters attacking the team's
methodology in 2005 — have been noticeably quieter
this time.
That is perhaps because the authors
made sure to address weaknesses pointed out in their
earlier work. The most important of these was reverse
causation - the possibility that some of the normal
and underweight subjects in the study were thinner because
they smoked or already had underlying chronic disease.
Complex subgroup analysis appears to rule out the possibility
that this is skewing the results.
These findings have been seized
upon with especial alacrity in the world's second-fattest
country, Britain, where an academic and public reaction
against the government's prophets of fatty doom is already
in full swing. Things got ugly this summer when the
government proposed extending sales tax to many food
types, ostensibly to control 'the obesity epidemic.'
The media naturally labelled it a 'fat tax.' By December,
government medics were in full retreat after it emerged
that a simple multiplication error had led them to wildly
overestimate future likely health costs of obesity.
The argument for weight loss is
likely to shift focus from the mortality burden of obesity
to morbidity. Indeed, the same issue of JAMA
that carried the mortality study also contained a research
article showing dramatically higher rates of disability
in the obese, and showing that the association between
obesity and disability has been growing in recent years.
A likely hypothesis, suggests a JAMA editorial
by Edward Gregg of CDC and Jack Guralnik of NIH, is
that with improved survival, the average obese person
today has lived with their extra weight for longer.
QUALITY
vs QUANTITY
But while some experts attack or defend the study, others
are likely to be largely unmoved by these findings.
Body mass index was already losing credibility as a
risk predictor. As Canada's leading expert in the field,
Jean-Pierre Després, the University of Laval
International Chair on Cardiometabolic Risk, recently
told the Canadian Cardiovascular Conference: "Some obese
people are healthy, yet others, with the same amount
of body fat, will have heart disease or diabetes. There
are simple ways to tell the difference but health professionals
are not yet using basic tools to identify who is at
greatest risk.
"A large waistline combined with
high levels of fasting triglycerides in the blood means
that there is about an 80% chance that the person has
metabolic syndrome," he went on, noting that the Quebec
Cardiovascular Study had shown a 20-fold increased risk
of ischemic heart disease in such patients. Visceral
fat is what counts, he maintains, and many agree.
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